1. Technical Field of the Invention
The present invention relates to a circuit used in the output stage of an operational amplifier.
2. Description of Related Art
An operational amplifier is constructed from a differential amplifier input stage, an intermediate high gain voltage amplifier stage and an output stage.
The output stage of operational amplifier circuits is designed to consume a low quiescent power and provide a high output power, providing an output voltage which can swing from rail to rail.
Output stages are classified into various types. Class A emitter follower stages output a voltage which varies linearly with the driving voltage, however this class of output stage often consumes a large quiescent current. Class B output stages are designed so that a source transistor and a sink transistor are connected so that both transistors cannot conduct at the same time. As with Class A output stages, Class B output stages provide a fairly linear signal response, however, Class B arrangements have crossover distortion at the point where both the sink and source transistors are off.
Class AB complimentary pair output stages are the most commonly used type of output stage. Class AB output stages comprise a source transistor and a sink transistor connected to the driving voltage with biasing circuitry. A Class AB output stage circuit is described in U.S. Pat. No. 5,825,228. This patent describes an output stage comprising a directly driven sink transistor and a sourcing transistor driven through a separate path. The sourcing path comprises an isolation transistor coupled to an NPN transistor configured as a common emitter amplifier that inverts the drive signal. A PNP transistor, connected to the collector of the NPN transistor across a diode, drives the source transistor. Since this configuration includes several common emitter stages in the path for controlling the sourcing transistor, the voltage gain will be higher when sourcing current.
It is an aim of the present invention to provide output stage circuits that provide a high output power, providing an output voltage which can swing substantially from rail to rail with low quiescent current and which overcome the disadvantages of the prior art.